The present invention generally relates to power amplifiers, and, more particularly, to a power amplifier which can control its output power to make it continuous, and to a power amplifier which can highly efficiently amplify modulated signals.
One conventional power amplifier is known in the art, in which a plurality of power amplifying units are used in parallel for performing efficient amplification, as shown in “Alireza Shirvani, David K. Su, and Bruce A. Wooley, “A CMOS RF Power Amplifier With Parallel Amplification for Efficient Power Control, “IEEE JOURNAL OF SOLID-STATE CIRCUITS. VOL. 37, NO. 6, JUNE 2002”.
FIG. 1 shows a basic structure example of a conventional power amplifier. This conventional power amplifier comprises a plurality of Class F amplifying circuits. Each of the Class F amplifying circuits comprises an FET (Field Effect Transistor), a λ/4 transmission line connected to a drain of the FET, a capacitor C, a tuning circuit, a load, an RF choke coil (RFC) for coupling DC power from a voltage Vdd to the FET, and a driver for supplying a required signal input level to the FET. By connecting the λ/4 transmission lines of all the Class F amplifying circuits together, the combined output signals from the Class F amplifying circuits make a transmission output signal.
Turning off any one of the drivers can prevent a corresponding Class F amplifying circuit from operating.
Each of the Class F amplifying circuits generally has input-output power characteristics (or input-output characteristics) and power efficiency (ratio of output power to DC power supply) to input power characteristics as shown in FIG. 2. When the output power approaches the saturation output power Ps, high efficiency is obtained, but the input-output characteristics becomes significantly non-linear. On the other hand, in an area where the input/output powers are small, the input-output characteristics become linear, but the efficiency is significantly degraded.
When the conventional power amplifier shown in FIG. 1 is used for amplifying constant envelope input signals, the non-linearity of the input-output characteristics does not provide a big problem. Therefore, when each of the Class F amplifying circuits operates near the saturation output power, high efficiency is obtained. In the conventional power amplifier shown in FIG. 1, by controlling the number of operating Class F amplifying circuits, a plurality of levels of transmission output signals can be obtained, that is, the transmission power can be controlled. High efficiency is obtained at different transmission levels.
Another prior art power amplifier is known, in which variable pre-attenuators for two parallel transistor amplifiers are controlled to reduce differences between the operating conditions of these two transistor power amplifying circuits, in order to prevent the mutual modulation distortion of the combined output signal due to transmission frequency variation in a radio transmitter, as shown in Japanese Patent Laid-Open Publication 2001-237651.
The conventional power amplifier shown in FIG. 1 operates well in providing highly efficient amplification and transmission power control of constant envelope signals. However, for amplitude-variable signals, transmission power levels become discrete, and there is a problem that a continuous output power level cannot be attained.
When power amplifying amplitude-variable signals using this conventional power amplifier, transmission output signals have large distortion due to non-linearity of each of the Class F amplifying circuits.
Further, even if the transmission output signals are averaged, the thus obtained average power levels are still discrete, and no continuous average power level can be attained.